Flicker noise, also known as 1/f noise, is a low frequency noise that is commonly observed in semiconductor devices such as MOS (metal-oxide semiconductor) field-effect transistors. As the name suggests, the power spectral density of a 1/f noise at a given frequency is inversely proportional to the frequency. Therefore, the power spectral density of the flicker noise is very high at a very low frequency. For applications where the signal of interest contains low frequency components, such as audio analog-to-digital converters and direct down-conversion receivers, one usually needs to use a low flicker noise operational amplifier, otherwise the desired signal could be spoiled by an excessively high flicker noise. A prior art differential operational amplifier is shown in FIG. 1 (bias circuits are not shown in the figure). This operational amplifier is a folded cascode amplifier comprising a bias transistor M0, a differential pair M1 and M2, a pair of cascode transistors M3 and M4, another pair of cascode transistors M5 and M6, a pair of P-type current sources Ip+ and Ip−, and a pair of N-type current sources In+, and In−. The basic idea of the folded cascode structure is to use a differential pair of transistors of one type to amplify the input differential signal, and use a pair of cascode transistors of the opposite type to boost the output resistance of the differential pair. In this folded cascode amplifier, the differential-pair transistors consisting of M1 and M2 are PMOS (P-type MOS) transistors, whereas the cascode transistors consisting of M3 and M4 are NMOS (N-type) transistors. This arrangement of using opposite-type cascode transistors allows its output to have larger swing ranges than using cascode transistors of the same type. Note that each of the two P-type current sources Ip+ and Ip− comprises a PMOS transistor, and each of the two N-type current sources In+ and In− comprises a NMOS transistor. In this operational amplifier, the two P-type current sources Ip+ and Ip− and the two N-type current sources In+ and In− are major contributors of flicker noise. What is needed is a method to alleviate the adverse effect of the flicker noise generated by these devices.